1. Field of the Invention
The invention concerns a method and an arrangement for keeping the nozzles of an ink print head clean, in particular of an ink print head having a large number of nozzles.
2. Description of the Prior Art
Ink print heads are provided for use in small, fast printers, which themselves are a component of modern machines for franking postal materials or for printing addresses or for product labeling.
In contrast to the usual office printers with line-by-line printing, the printing ensues as a unique impression in one passage of the print medium. Due to this necessarily larger printing breadth (approximately an inch), the number of ink nozzles to be arranged under one another is substantially more than in ink print heads for office printers. In order to fulfill current customer desires (blocks with word and image characters) for franking machines with good print quality, print resolutions of close to 200 dpi (dots per inch) are required. This means ink print heads with 200 nozzles.
In these print heads it must be ensured that relevant security-oriented print image data, such as value, date and machine number for franking machines, are printed without omission of print points (dots).
Since fast-drying inks must be used, there is a high risk that the ink will dry in the nozzles that are not used (activated) for a longer time, or that the nozzles will become clogged by the accumulation of dust and ink residue in the area of the nozzles.
For preventing or remedying clogging of the nozzles, a number of different solutions is known, such as uncovering and vacuuming of the nozzle surface of the ink print head, expelling ink through all nozzles, wiping the nozzle surface with a wiping lip, and supplying cleaning agents to the nozzle surface; see German OS 38 10 698 and European Application 0 285 155.
All these measures have in common that they ensue exclusively during print pauses, whose spacing can be far apart in time from one another in printers that print frequently and for long durations, such as franking machines. Clogging is accordingly probable.
An ink jet printer with several nozzles is known (German OS 33 11 735) in which detection means registers when a nozzle or several nozzles are in an unused state for a predetermined time period, and in which the result of the detection is evaluated by control means, and ink is caused to be expelled through the nozzles concerned. In other words, the time duration of the print pauses is measured by signal measurement means, and if the print pause exceeds a predetermined time limit, rinsing of the nozzles used for printing is carried out by rinsing means before printing. In order to be fairly certain that the nozzles also remain truly capable of functioning, the print pauses must be relatively short; accordingly, the writing operation must be interrupted for the purpose of cleaning. Thus the operating time of the printer is reduced and the ink consumption is essentially increased by this cleaning method.
Furthermore, a cover for the nozzles of a partial-vacuum ink print head operating with fast-drying inks is known; cf. European Application 0 173 939. By means of the cover, it is intended that the ink be prevented from drying at the nozzle ends in the print pauses, while it is also intended to ensure that the ink meniscus is neither touched nor pushed back.
For this purpose, the cover has a circumferential sealing element and a membrane, which form a pressure chamber when the cover is set on the nozzle surface. The internal tension of the membrane is smaller than the surface tension of the ink meniscuses. In this relatively small pressure chamber, a rapid saturation of the air with moisture occurs as a result of ink solution agents that evaporate at first, so that drying out of the nozzles is avoided over shorter print pauses. This arrangement, however, does not offer protection against nozzles drying out that are not used for a longer time during operation.
Another known apparatus (cf. German OS 38 25 045 and German OS 38 25 046) for cleaning the nozzle surface of an ink print head has a wiping element arranged movably in front of the nozzle surface, and in addition to the ink nozzles has a nozzle from which cleaning fluid is expelled against a guard screen and is diverted from this screen onto the nozzle surface. The wiping element is a belt that contains a number of openings adjacent to one another in the direction of the belt, the belt being transported past the nozzle surface in one direction. According to the position of the belt, the nozzles are released (opened) or covered. The belt is fashioned as an endless belt and is contained in a belt cartridge with a drive. During print operation, the belt lies with one of its openings in front of the nozzle openings and releases these openings for the expelling of ink. In the transition from print operation to print pause, at first cleaning fluid is expelled drop by drop through the cleaning nozzle. Subsequently the belt is moved forward in its direction of transport so far that instead of the opening, the following section of the belt moves in front of the nozzle surface and covers it. The edge of the opening wipes over the nozzle surface and thereby cleans this surface of accumulated contamination.
As is clear, this apparatus serves only for a rough cleaning of the nozzle surface. Clogging of the nozzle openings as the belt slides by is not precluded, nor is a drying out of nozzles.
In another known method (cf. German 32 03 014), an air cell is positioned in front of the nozzle opening for preventing clogging of the nozzles of an ink printer. This cell has an outer opening aligned with the nozzle opening, through which outer opening the ink drops fly onto the print medium. During operation, the ink jet is accompanied by an air jet from the air cell that annularly surrounds the ink jet, and is thereby accelerated. During a print pause, in order to keep the nozzle moist, the outer opening of the air cell is closed by means of a porous covering body and the moisture in the air cell is increased by letting ink into the air cell while the air is evacuated from the air cell.
At the beginning of the print operation, the ink in the air cell is rinsed out of this air cell through the outer opening by means of increased air pressure. For this purpose, the covering body is turned away, and a receptacle holder for the ink is brought in front of the outer opening. After emptying the air cell, the outer opening is blown free of ink by the air streaming through. The receptacle holder is removed and the print medium is brought into position.
The apparatus for accomplishing this known method includes an impulse-actuated pressure cell, an ink cell pressure-coupled with the pressure cell and connected to an ink cartridge via an ink duct, an air cell that is divided from the ink cell by a wall having a first opening and that is connected with the atmosphere by an outer opening, and a pump that during operation is connected with the air cell via an air line and that places the air line under a partial vacuum when operation is not taking place.
A cover apparatus can be positioned in front of the outer opening, and a valve is contained in the air line that relieves the pressure in the air line during non-pressure operation, and in preparation for the print operation raises the pressure so far that the ink contained in the air cell is pressed out through the outer opening. A receiving container, adjustable in two positions, is arranged for collecting the ink pressed out of the air cell.
An ink level state sensor in the air line switches off the supply of ink if the ink level reaches the sensor.
Although it is advantageous to integrate a part of the cleaning apparatus in the ink print head, this solution still has some disadvantages. Protection against drying out and clogging ensues in this known apparatus only during print pauses. The air stream during operation can lead to a faster drying out, at least for the nozzles that are seldom or not at all used. Since the printing ink is also used to maintain moisture in the print pauses and is then pressed into an external receptacle holder each time before printing begins, this procedure is costly with respect to time; moreover, ink is lost each time. Also, the ink meniscus can be pushed back to an undesirable extent.
Moreover, the mechanical expense (a covering body as well as a receptacle holder that can be positioned outside the ink head, and also a pump with a valve and a sensor inside the ink head) is considerable.